Early biological warfare was used at the Mongol siege of Caffe, a Crimean city which belonged to the Genoese. When the Mongol army became decimated by the bubonic plague, they realised they would have to pack up their siege and leave. Before doing so, they catapulted infected corpses over the walls. The effect on world history was enormous. The Genoese sailed back to Genoa, infecting city after city along the way. Within a few years, virtually the whole of Europe was infected.